This invention relates to an apparatus for monitoring the thread drawn off an open end spinning turbine which is driven with a predetermined rpm by a motor, particularly a brushless DC motor. The apparatus has a measured value transmitter which responds to variations of the current consumption of the motor and generates a signal when the current variation exceeds a predetermined threshhold value.
In U.S. patent application Ser. No. 863,156, filed Dec. 22, 1977, now U.S. Pat. No. 4,158,284, it has been proposed to utilize the variations of the motor current to detect thickened yarn portions caused by a localized soiling of the rotor groove or to detect variations in the yarn structure caused by groove wear or a wide-spread deposition of dirt in the groove. As the yarn is drawn off the groove, yarn irregularities, for example, thickened locations cause, because of the altered yarn mass (change in friction) and because of the altered (increased) air resistance of the yarn, a change of motor torque and thus a change in the current consumption of the spinning motor.